1. Field of the Invention
The present invention relates to a wafer transfer system. More particularly, the present invention relates to a wafer transfer system for use in a semiconductor manufacturing process and a method of controlling pressure in the wafer transfer system by varying air flow into and out of the system.
2. Description of the Related Art
During semiconductor processing, wafers are required to be transferred from one location to another location. For example, wafers are required to be transferred from a wafer container into process equipment. A wafer transfer system is necessary to perform this transfer. In view of the high degree of cleanness that must be provided in semiconductor manufacturing, all areas in the semiconductor manufacturing process to which the wafers are exposed, including the wafer transfer system, must be maintained at a very high level of cleanness.
By effectively maintaining a high degree of cleanness in the wafer transfer system, a processing wafer and a wafer container can avoid initial contamination or re-contamination by a contaminated wafer container or wafer transfer system.
Conventionally, to eliminate contamination during a semiconductor manufacturing process, the manufacturing process was performed in a clean room maintained in a high degree of cleanness and an open type container was used to store and transfer wafers.
In an effort to reduce maintenance costs of the clean room, an equipment part of the clean room is selectively maintained in a relatively high degree of cleanness, while the remainder of the clean room is maintained in a relatively low degree of cleanness. Accordingly, it is necessary to use a sealed type wafer container to prevent the wafers from becoming contaminated when the wafers are in an area having a relatively low degree of cleanness.
The equipment part of the clean room includes a wafer transfer system for transferring wafers into the process equipment from the wafer container. The wafer transfer system includes a housing maintained locally in a high degree of cleanness. The housing includes a blowing fan in an upper area thereof for blowing clean air downwardly and an exhaust valve in a lower portion thereof for exhausting air.
During a transfer process in the wafer transfer system, particles are generated in the housing by operation of a wafer transfer robot. These particles attach in slots in the wafer container due to a flow of air when the wafer container is opened or closed. A processed wafer is contaminated by particles attached onto the slots, and other wafers (not processed) are also contaminated in the housing before being transferred into the process equipment.
One example of particle contamination is nano-sized particles, i.e., particles having a size on the order of nanometers. Nano-sized particles, or nanoparticles, require a high blowing speed of the fan to remove the particles. An amount of electricity consumed by the fan rotating at a high speed in an effort to remove nanoparticles is extremely high. Moreover, when a hole size of a conventional exhaust valve is not sufficiently large, not all of the nanoparticles are sufficiently exhausted. Resultantly, the nanoparticles accumulate adjacent to the exhaust valve and eventually move up to the top area of the housing. This insufficient exhausting results in a “whirlpool effect” of air and particles in the housing.
Conventional wafer transfer systems have attempted to increase a speed of the blowing fan to remove additional particles in the housing. However, significant electricity is consumed due to the high speed and continuous rotation of the blowing fan. In an effort to reduce electricity consumption, other conventional wafer transfer systems have operated a fan at two speeds. However, when a blowing speed of the fan is reduced to conserve energy, sufficient differential pressure between the housing and the clean room may be lost and contamination may enter the housing.